1. Field of the Invention
The present invention relates to a method for fabricating a preform for a plastic optical fiber and a preform fabricated thereby. Particularly, the present invention relates to a method for fabricating a preform for a plastic optical fiber having a refractive index gradient in a radial direction and no vacancy therein wherein additional monomer or prepolymer is charged into the vacancy formed after polymerization so as to compensate for a volume shrinkage due to polymerization of the monomer in a centrifugal field and then is reacted.
2. Description of the Related Art
Optical fibers used in the field of telecommunications are generally classified into a single-mode fiber and a multi-mode fiber in terms of the transmission mode of optical signal. Optical fibers currently used for long distance, high speed communications are mostly the step-index, single-mode optical fibers based on quartz glass. These optical fibers have a diameter as small as 5 microns to 10 microns, and as a result, these glass optical fibers face serious challenges in terms of achieving proper alignment and connection Accordingly, these glass optical fibers are associated with expensive costs related to achieving proper alignment and connections.
Alternatively, multi-mode glass optical fibers having a diameter that is larger than the diameter of single-mode optical fibers may be used for short distance communication such as in local area networks (LANs). However, these multi-mode glass optical fibers, in addition to being fragile, also suffer from expensive costs related to achieving proper alignment and connection and therefore are not widely used. Accordingly, these multi-mode glass optical fibers have been mainly used for short distance communication applications up to 200 meters such as in LANs using a metal cable, for example, a twisted pair or coaxial cable. However, since the information transmission speed or bandwidth of the metal cable is as low as about 150 Mbps and cannot reach transmission speeds of 625 Mbps, which is a standard for the year 2000 in accordance with Asynchronous Transfer Mode (ATM), it can not satisfy the future standard of transmission speed.
To cope with these problems, the industry has expended great effort and investment over the past 10 years towards development of polymer optical fibers, which can be used in short distance communication applications, such as LANs. Since the diameter of polymer optical fibers can be as large as 0.5 to 1.0mm which is 100 or more times than that of glass optical fiber, due to its flexibility, its alignment and connection are much easier issues than with the glass optical fibers. Moreover, since polymer-based connectors may be produced by compression molding, these connectors can be used both for alignment and for connection and thereby reduce costs.
On the other hand, the polymer optical fiber may have a step-index (SI) structure, in which a refractive index changes stepwise in a radial direction, or a graded-index (GI) structure, in which a refractive index changes gradually in a radial direction. However, since polymer optical fibers having a SI structure have high modal dispersion, the transmission speed (or bandwidth) of a signal cannot be higher than that of cable. On the other hand, since polymer optical fibers having a GI structure have low modal dispersion, it can have a high bandwidth. Accordingly, since GI polymer optical fiber can be fabricated at a reduced cost due to its larger diameter, the bandwidth can be maintained as high as possible on account of low modal dispersion Therefore, it is known that GI polymer optical fiber is adequate for use as a communication medium for short distance, high-speed communication because of reduced costs derived from its larger diameter and high speed of information transmission derived from low modal dispersion.
The conventional method for fabricating GI polymer optical fiber was first reported by a Japanese professor, Koike Y. et al. of Keio University in 1988 [refer to xe2x80x9cKoike, Y. et al., Applied Optics, Vol. 27, 486 (1988)xe2x80x9d]. Since then, other related-techniques have been disclosed in U.S. Pat. No. 5,253,323 Nippon Petrochemicals Co.); U.S. Pat. No. 5,382,448 (Nippon Petrochemicals Co.); U.S. Pat. No.5,593,621 to Yasuhiro Koike and Ryo Nihei; WO 92/03750 (Nippon Petrochemicals Co.); WO 92/0375 1; Japanese Patent Laid-Open No. 3-78706 (Mitsubishi Rayon Co., Ltd.); and Japanese Patent Laid-Open No. 4-86603 (Toray Ind.). The methods disclosed in these prior patents are mainly classified into two methods as follows.
The first method is a batch process wherein a preliminary molding product, namely a preform in which a refractive index changes in a radial direction, is fabricated, and then the resultant preform is heated and drawn to fabricate GI polymer optical fiber.
The second method is a process wherein a polymer fiber is produced by extrusion process, and then the low molecular material contained in the fiber is extracted, or contrarily introduced in a radial direction to obtain GI polymer optical fiber.
It is known that the first method directed to a batch-type process introduced by professor Koike can successfully fabricate a GI polymer optical fiber having a transmission speed of 2.5 Gbps, and that the second method could also successfully fabricate a polymer optical fiber having a relatively high bandwidth.
Van Duunhoven reported another method for fabricating GI polymer optical fiber wherein when the monomers having different density and refractive index are polymerized in a centrifugal field, a concentration gradient is generated on account of a density gradient, and thus the refractive index gradient is generated as disclosed in WO 97/29903. In other words, when two kinds of monomers having different density and refractive index are polymerized in a centrifugal field, a concentration gradient is generated by a density difference, and a refractive index gradient is generated by the concentration gradient, provided that the refractive index of the monomer with high density is higher than that of the monomer with low density.
However, Van Duunhoven did not address nor mention anything relating to the problem inevitably caused by volume shrinkage. In other words, since volume shrinkage occurs when monomers are polymerized (to produce a polymer), there is a void that forms in the middle of the perform. In other words, the preform for a plastic optical fiber fabricated by centrifugal force is hollow or vacant in its center to form a shape of a tube. Accordingly, when the optical fiber is fabricated by using the volume-shrunk preform, a discontinuity of the refractive index profile appears to the level or degree as the amount of void, hollow or vacancy, which can lead to a significant or remarkable reduction in the level of transmission, so much so that the optical fiber may not be useable.
Alternatively, when a preform for a plastic optical fiber having a large volume is fabricated by a conventional method, since it has a tubular shape due to a volume shrinkage and a high speed revolution of the preform, the resultant plastic optical fiber deteriorates in quality.
As an additional general drawback it is difficult to fabricate a preform for a plastic optical fiber at a low cost.
A feature of a preferred embodiment of the present invention is to provide a method for fabricating a preform for a plastic optical fiber wherein an additional monomer and/or prepolymer is charged into a void or hollow or vacancy formed due to or on account of the volume shrinkage generated upon producing the preform in a centrifugal field, and then polymerizing the additional monomer and/or prepolymer to prevent the formation of a discontinuous refractive index profile in a radial direction.
Another feature of a preferred embodiment of the present invention is to provide a preform for a plastic optical fiber fabricated by the method of the present invention. According to one aspect of a preferred embodiment of the present invention, there is provided a method for fabricating a preform for a plastic optical fiber comprising the steps of:
(a) introducing a mixture consisting of at least two substances, each having a different density and refractive index relative to each other, in a cylindrical reactor and polymerizing the mixture in a centrifugal field generated by rotating the reactor;
(b) compensating for a void formed by volume shrinkage from the polymerization in step (a) with a mixture consisting of at least two substances, each having a different density and refractive index relative to each other, and polymerizing the mixture in a centrifugal field generated by rotating the reactor; and
(c) repeating step (b) until there is no void after polymerization.
According to another aspect of a preferred embodiment of the present invention, there is provided a preform for a plastic optical fiber that is made by the above method.